In the conventional manufacture of rubber footwear products, uncured rubber or rubber coated fabrics are typically milled or extruded into appropriate thicknesses from which the desired sized component parts for the footwear product are cut. The uncured components are normally assembled upon a forming device such as a metal last. The metal last bears the internal shape and size of the desired footwear product. The last is normally a heat conductive metal form such as a hollowed, shoe or boot-shaped aluminum form. It is conventional to assemble on the forming last an uncured rubber upper, an uncured rubber insole, an uncured foxing band (e.g. a flat uncured strip), an uncured rubber binder (e.g. uncured sheeting stock or filler) and a precured rubber sole, which has been coated upon the forming last, to form a footwear assembly. This completed footwear assembly is then vulcanized to provide the desired rubber footwear vulcanized product.
An uncured, rubber shoe or boot upper is typically positioned to adhesively overlap the insole upon the last. An uncured foxing band or flat band (e.g. 1-2 inches in width) is then placed around the outer periphery or rim of the rubber upper. An uncured binder or filler is then placed onto the insole. The binder or filler will not normally extend beyond the exposed surface of the insole. An uncured rubber outsole is then wrapped over the projecting edge of the insole onto the uncured rubber upper above the feather line. This uncured rubber assembly, which has the appearance of crude footwear, is then vulcanized upon the last, typically for about 1 to 2 hours at temperatures ranging from about 250.degree.-350.degree. F., to provide the vulcanized product. The component parts are thereby chemically and physically melded into a complete vulcanized footwear product of unitary construction.
Uncured (unvulcanized) natural or synthetic rubber footwear components normally possess sufficient tack for assembly upon the forming last. An uncured rubber formulation generally includes ethylenically unsaturated polymeric substances in combination with conventional vulcanization or curative additives. Included amongst such conventional additives are crosslinking agents such as sulfur, accelerators, tackifiers, antioxidants, and activators. The additives and polymeric rubber substances are usually admixed in a Branbury mixer and then formed into the desired thickness or shape of the footwear component parts. This is usually accomplished by milling, extruding, and calendering techniques, without curing the rubber formulation.
Vulcanization of the uncured footwear assembly alters the chemical and physical attributes of the rubber composition by crosslinking ethylenically unsaturated polymeric linkages. Crosslinking occurs not only within each of the individual assembled component parts, but also between ethylenically unsaturated polymeric linkages positioned at the interface of the adhesively overlapped component parts. This melds the component parts into a unitary, vulcanized product.
Heretofore, conventional manufacture of such rubber footwear products has required vulcanization of the entire footwear assembly. This requirement has placed severe constraints upon the rubber footwear manufacturing industry. Thermoplastic and other natural or synthetic polymeric substances thermally sensitive or degradable upon vulcanization are not acceptable for use in such a vulcanized product. Outsoles for such footwear products are essentially limited to vulcanized rubber outsoles. Such constraints have made it difficult for the rubber footwear industry to effectively compete against other footwear products capable of being produced in more fashionable designs (e.g. high heeled shoes, western boots, contrasting sole colors, etc.) at low cost. Manufacture of products such as footwear of a lightweight construction, replaceable or repairable soles, specialty soles adapted for specific uses (e.g. spiked, cleated, or ridged soles for sports such as golf, football, hiking, baseball, etc.) and products having other specialty wear features is not feasible using existing rubber footwear manufacture technology.
It would be desirable to incorporate many of the desirable attributes of such other, non-rubber footwear products into a footwear vulcanizate while retaining many of a vulcanizate's desirable attributes, such as resistance to air, gas, sunlight, hydrocarbons, moisture penetration, fats and oils, and acid and other chemicals, together with its excellent durability, strength, elasticity, electrical and heat insulating properties, and structural integrity. A different method of manufacturing rubber footwear products would be desirable if it could provide footwear products uniquely different in construction, utility and design from conventional rubber footwear. Vulcanized rubber footwear products able to separately receive an outsole would afford significant labor, equipment, production time and material savings. Notwithstanding a long felt need for such rubber footwear products, the manufacture of rubber footwear vulcanizates has remained essentially unchanged since the beginning of the 20th century.
Athletic sports footwear products (e.g. tennis shoes, etc.) generally differ from products of vulcanized rubber manufacture in that such sports footwear frequently include a polyurethane outsole. Such outsoles are normally incorporated into the athletic footwear product by injection molding. U.S. Pat. No. 4,245,406, by Landay, discloses a sport shoe having an outsole comprised of an injection molded polyurethane midsole to which a rubber outer sole is attached. Similarly, U.S. Pat. No. 4,455,765, by Sjosward, discloses a sport shoe product having an outsole with a polyurethane midsole and a rubber outer sole. A patent issued to Giese, U.S. Pat. No. 4,366,634, discloses an injection molded sport shoe, such as a tennis shoe, having a fabric upper portion secured to a sidewall and an outsole constructed of two different materials (preferably rubber and polyurethane midsole) to which a rubber outer sole is bonded. Another variation involving polyurethane shoe construction is disclosed by U.S. Pat. No. 4,228,600 by Kruge. The Kruge shoe bottom comprises a flexible, closed-cell, crosslinked polyolefin foam having at least one surface of exposed open cells and an elastomeric polyurethane film which encompasses the foam.
A method for manufacturing a ski boot constructed of an upper part and a prefabricated sole is disclosed in U.S. Pat. No. 3,273,263 by Kilma. The prefabricated sole of the Kilma patent includes a metal plate adapted to receive notched nails or screws embedded within the sole. The sole is secured to the ski boot by means of glue and nails or screws. In another U.S. patent by Denu, U.S. Pat. No. 4,130,947, a method of making a sole is disclosed wherein a layered substance resistant to abrasion is vulcanized onto a dampening layer of a synthetic or natural rubber.
U.S. Pat. No. 2,983,643 by Seiberling discloses a heel ostensibly suitable for adaptation to footwear constructed from leather uppers. Seiberling mentions, at column 1, lines 25-32, the difficulties typically encountered when adhesively bonding a cured rubber heel to another surface. According to Seiberling, it is customary to abrade the skin surface of the cured rubber to roughen it. Otherwise, the cured skin becomes so slick that it does not form a good bond with the adhesive. The roughened surfaces of the cured rubbers may then be adhered to another surface by an adhesive. Seiberling proposes to avoid the need for roughening by incorporating cellulosic fibrous materials into that portion of the heel surface to the glued (e.g. by animal glue) onto the shoe. This involves placing an uncured or raw rubber biscuit with an overlying fibrous mat into a mold, then vulcanizing to form a heel vulcanizate having embedded cellulosic fibrous projections upon the glueing side.
A European patent specification by Phillips (Publication No. 0 075 861) further illustrates the problem with repairing leather footwear using thermosetting butadiene-styrene half soles. Phillips proposes to overcome this problem by imparting a pebbled surface texture onto the half sole. The pebbled surface reportedly enhances the adhesive properties of the sole and facilitates application of a hot melt adhesive thereto.